1. Field of the Invention
This invention relates to a process of freezing sushi, boiled rice or processed food with boiled rice as the main component.
Among varieties of sushi which can be frozen by a process according to the invention is nigiri-sushi (or edomae-sushi) 1 as shown in FIG. 19(A). A piece of this food is prepared by forming an adequate amount of boiled rice 1a with vinegar into a mouthful-sized oval piece, putting a piece of fish, shellfish, shrimp, etc. 1b on this rice piece, and then adjusting the overall shape of the food by lightly gripping the same. Another variety of sushi is oshi-sushi 3 as shown in FIG. 19(B). This food is prepared by filling a rectangular wooden frame of about 20 cm.times.10 cm with boiled rice with vinegar 3a, putting a piece or pieces of fish, shellfish, shrimp, etc. 3b on the rice, pressing the resultant food from above with a wooden lid for shape adjustment, and then cutting the food into mouthful-sized pieces. A further variety of sushi is inari-sushi 4 as shown in FIG. 19(C). This food is prepared by using aburage (or fried sliced tofu) 4b prepared by frying rectangular sliced tofu in oil after water draining. Each aburage piece 4b is cut into two halves, and these aburage halves are boiled using sweet shoyu. Then, each boiled aburage half is opened from the cut side into a sack-like form, and then boiled rice 4a with vinegar is charged into the open boiled aburage half. In this way, a piece of inari-sushi is obtained. A further variety of sushi is maki-sushi or norimaki 12 as shown in FIG. 19(B). As shown, a piece of this food has a circular sectional profile. This variety may also include futomaki 13. This variety comprises a central ingredient part 12b, 13b which may be a piece or pieces of fish, baked egg, cucumber or like vegetables, etc., a boiled rice part 12a, 13a surrounding the ingredient part and having a predetermined thickness, and paper-like toasted layer 12c, 13c surrounding the boiled rice part. (The norimaki 12 and futomaki 13 are different in that in the norimaki 12 only a single kind of ingredient is used for the central ingredient part, while the futomaki 13 uses a plurality of different ingredients, the former being about 3 cm in diameter and the latter being about 5 cm.)
Particularly, the invention concerns a process of freezing sushi, which permits substantially the same quality as non-frozen sushi to be obtained after natural thawing.
Further, the invention is applicable to a process of freezing boiled rice or processed food, with boiled rice as a main component, permitting substantially the same quality as non-frozen food to be obtained after natural thawing. Among varieties of this type of food, there is boiled cleaned or uncleaned rice as shown in FIG. 20(A). The boiled cleaned rice 5 is prepared by boiling cleaned rice or white rice grains, which are obtained by polishing off surface portions of uncleaned rice after hulling. The boiled uncleaned rice is prepared by boiling uncleaned rice in a high pressure oven. A further variety of the food is sekihan 6 as shown in FIG. 20(B). This food is prepared by boiling or steaming glutinous rice with azuki bean, making the rice itself pink. A further variety of the food is rice cake 7 as shown in FIG. 20(C). This food is prepared by squeezing steamed glutinous rice into a grain-free state. A further variety of the food is onigiri 8 utilizing boiled rice as shown in FIG. 21(A). This food is a substantially triangular piece of boiled rice 8a formed with both hands or with a wooden frame. If desired, pickled fish egg, pickled plum, etc. may be provided inside. Further, onigiri may have a cover of a paper-like layer 8b. A further variety of the food is takikomi gohan (combination boiled rice) 9 as shown in FIG. 21(B). This is prepared by boiling rice together with sliced meat, vegetables, fish, shellfish, etc. and also with shoyu or like seasoning. A further variety is fried rice prepared by frying boiled rice with slices of meat, vegetables, fish, shellfish,. etc. A further variety of the food is okowa (combination steamed glutinous rice) similar to the takikomi gohan 9. This food is prepared by steaming glutinous rice together with meat, vegetables, fish, shellfish, etc. and also with shoyu or like seasoning. Sometimes, the materials are steamed while they are wrapped in bamboo bud skin. A further variety of the food is pilaf which is prepared by boiling rice together with meat, vegetables, fish, shellfish, etc. and also with olive oil, Safran or like seasoning. A further variety of the food is ohagi 11 as shown in FIG. 22(A). This food is a mouthful ball-like boiled glutinous rice piece 11a in sweet boiled azuki bean 11b. A further variety of the food is chirashi as shown in FIG. 21(C), which is prepared by mixing boiled rice with vinegar and sliced fish, baked egg, vegetables such as cucumber, etc. A further variety of the food is beaf bowl or other bowls, which is prepared by filling a bowl with rice and then putting cooked meat, egg, curry, etc. on the rice. There are further varieties of the processed food, the volume of which is mainly occupied by boiled rice, for instance curry rice.
2. Description of the Prior Art
Among recent frozen foods are fresh foods and cooked foods, and boiled rice, rice cake, sushi and the like are not exceptions. These conventional frozen foods, however, have their texture destroyed when they are frozen. Therefore, when they are thawed, a great deal of liquid is produced, giving rise to great differences of smoothness and sense of touch compared to fresh foods.
Particularly, frozen nigiri-sushi presents such problems as escapement of vinegar from the rice ball part, which is made from boiled rice with vinegar and carries a piece of fish or the like, hard rice grains due to loss of stickiness, discoloring of ingredients, loss of taste, etc.
Also, boiled rice or processed food with boiled rice as a main component, when frozen and thawed in the usual way, results in hard rice grains due to loss of stickiness and therefore in unsatisfactory taste, sense of eating, etc.
To solve the above problems, various processes of freezing sushi or boiled rice or the like have been proposed.
For example, Japanese Patent Laid-Open Publication No. Sho 61-260843 (Prior Art Example 1) shows a freezing process in which maki-sushi is frozen quickly in a state of being wrapped in transparent film to a temperature of -40 to -70.degree. C.
Also, Japanese Patent Laid-Open Publication No. Hei 2-100643 (Prior Art Example 2) shows a method of manufacturing frozen sushi in which washed rice is boiled while adding vinegar to additive liquid for extraction of branched cyclodextrine for vinegar escapement prevention and organic phosphate, amino acid, etc. for preventing the hardening and oxidation of the rice to obtain boiled rice with vinegar. Sushi is then prepared with the boiled rice and ingredients, and the sushi thus prepared is frozen momentarily in contact with liquid nitrogen gas in a short period of time (i.e., 18 to 20 min.).
Further, Japanese Patent Laid-Open Publication No. Sho 5-38266 (Prior Art Example 3) shows a process of producing frozen boiled rice including frozen sushi, in which rice boiled in the usual way is cooled down to 25 to 40.degree. C., formed to a predetermined shape, frozen quickly in a freezer preliminarily cooled down to -40 to -50.degree. C. by blowing liquified gas against the rice while maintaining a constant reduced pressure in the freezer, and then frozen continually by passing a maximum ice generation temperature range in a short period of time (i.e., 3 min.), thus obtaining substantially the same temperature of the superficial and central parts of the formed rice.
Prior Art Example 1, however, is a special process of freezing maki-sushi and can not be applied to nigiri-sushi with a piece of fish, shellfish, cooked egg (hereinafter referred to as sushi raw materials) put on a riceball. Bedsides, the process requires wrapping each sushi piece, which is very cumbersome.
Prior Art Example 2 requires especially prepared additives or additive liquids for preventing escapement of vinegar from boiled rice with vinegar and the hardening and oxidation of the boiled rice. The tastes of such additives cause bitterness, leading to great deterioration of the taste.
In either of the above techniques, a significant problem is posed by the quick freezing sushi when passing the maximum ice generation temperature range (-1 to -5.degree. C.) in a minimum period of time for texture destruction prevention in the freezing process. A freezing curve which is obtainable when quickly passing the maximum ice generation temperature range for quick freezing, for the rice ball part as shown in FIG. 3, does not always provide for good taste, and the quality of food is extremely deteriorated compared to non-frozen sushi.
A further problem which is posed in case of frozen sushi is that the technique of thawing is usually very difficult. This is because the sushi raw material part (e.g., fish piece) and rice ball part require different thawing times when using an electronic oven, for instance, for thawing. Therefore, setting the thawing time for the raw material results in most of the rice ball remaining in the frozen state. If the thawing time is set for the rice ball part, on the other hand, the sushi raw material is excessively heated by the heat of the electronic oven during the thawing of sushi.
Accordingly, a technique for high frequency heating sushi raw materials when wrapped in aluminum foil is disclosed in Japanese Patent Laid-Open No. Sho 63-24864. However, it is practically impossible to separate the sushi raw material part and rice ball part of the frozen sushi. Basically, therefore, a freezing technique which permits sushi obtained by natural thawing to have the same quality as non-frozen sushi is necessary.
It has been proposed to vacuum seal boiled cleaned rice or the like and water in a resin film bag for steaming under pressure and subsequent freezing and thawing; this is disclosed in Japanese Patent Laid-Open Publication No. Sho 60-16560. Such a freezing process, in which rice is steamed under pressure when packed with water, is not applicable to onigiri or like processed food.
Further, what is commonly called a loose rice freezing process, in which boiled rice is frozen in a loose state such that the individual grains are distinct from one another, is disclosed in, for instance, Japanese Patent Laid-Open Publication No. Sho 62-253350. Such a freezing process provides for rice with individual grains distinct from one another due to lack of stickiness. Such rice, although suitable as the material of pilaf, is unsuited for eating directly after thawing. Further, the rice cannot be sold directly after thawing as a product in shops.
Further, a technique of freezing rice that has been boiled with an organic acid or a derivative thereof is disclosed in Japanese Patent Laid-Open No. Sho 60-172262. According to such a technique, the taste is deteriorated by the additive.